This invention relates to producing oil from relatively deep viscous oil reservoirs such as tar sands, or the like. More particularly the invention relates to improving the efficiency with which such a reservoir is heated and oil is produced by utilizing horizontal wells which are interconnected by vertical fractures.
In tar sand deposits, there is frequently little possibility of injecting significant quantities of fluid. Although such reservoirs may have a high absolute permeability, due to a high tar saturation and viscosity and a low water saturation, the effective permeability may be very low at the reservoir temperature. In shallow deposits it is usually feasible to fracture a reservoir and interconnect wells by means of horizontal fractures. In thick, shallow reservoirs, overlapping pairs of such horizontal fractures can be utilized in a steam drive process of the type described in my U.S. Pat. No. 3,129,758.
However, in deep earth formations, hydraulic fractures are preferentially vertically oriented, particularly at depths significantly greater than about 1,000 feet. In general, fractures tend to be aligned perpendicular to the least compressive stress within the formation. In the deeper reservoirs, the vertical compressive stress due to the weight of the overburden is usually the greatest. Therefore, hydraulic fractures are preferentially vertical fractures aligned along a horizontal direction dictated by the local tectonics of the region.